star wars - Why do battle droids need to speak in Basic?

In-universe, the most likely reason for programming the battle droids to speak in Basic is communication with non-droid personnel.

In several scenes, we see battle droids interacting with non-droids - usually their Nemoidian masters & operators. Here's an example from Episode I, but there are plenty throughout the prequel trilogy movies:

The droid invasion force moves out of the swamp and onto a grassy
plain. OOM-9, in his tank, looks out over the vast ARMY marching
across the rolling hills. A small hologram of RUNE and NUTE stands on
the tank.

OOM-9: Yes, Viceroy?

RUNE: Captain, we've searched the ship and there is no trace of the
Jedi. They may have got on onto one of your landing craft.

OOM-9: If they are down here, sir, we'll find them.

NUTE: Use caution. These Jedi are not to be underestimated.

While it would seem to make sense for droids to communicate with each other via some sort of binary language or wireless communication, it's possible that the Trade Federation simply cut costs by giving them a single, verbal communication method. Since they needed to communicate in Basic anyway, perhaps that was deemed enough and warranted no further research/spending.